The present invention relates generally to composites having a titanium base matrix reinforced by silicon carbide fiber or filament reinforcement. More particularly, it relates to improvements in the matrix components of a silicon carbide reinforced titanium aluminide composite.
The preparation of titanium alloy base foils and sheets and of reinforced structures in which silicon carbide fibers are embedded in a titanium base alloy are described in the patents: U.S. Pat. Nos. 4,775,547; 4,782,884; 4,786,566; 4,805,294; 4,805,833; and 4,838,337; assigned to the same assignee as the subject application. The texts of these prior art patents are incorporated herein by reference. The preparation of these composites is the subject of intense study inasmuch as the composites have very high strength property in relation to their weight. Prior to the development of the processes described in the above-referenced patents, such structures were prepared by sandwiching the reinforcing filaments between foils of titanium base alloy and pressing the stacks of alternate layers of alloy and reinforcing filament until a composite structure was formed. However, that prior art practice resulted in some misalignment of the reinforcing fibers.
The structures taught in the above-referenced patents greatly improved over the earlier practice of forming sandwiches by compression.
It has been found that while the structures prepared as described in the above-referenced patents have properties which are a great improvement over earlier structures, the attainment of the potentially very high ultimate tensile strength in these structures did not measure up to the values theoretically possible.
The testing of composites formed according to the methods taught in the above patents has demonstrated that although modulus values are generally in good agreement with the rule of mixture predictions, the ultimate tensile strength is usually much lower than predicted by the underlying properties of the individual ingredients of the composite. Further, testing has shown that the total strain to composite fracture is relatively low and, in addition, extensive off-plane cracking of the matrix has been observed It has been found that the matrix in composites formed with SiC reinforcement in a Ti-1421 matrix consist primarily of alpha-2 crystal form which is an ordered intermetallic phase and the secondary constituent of the matrix is small amounts of beta-phase. The alpha-2 crystal material tends to have low ductility and envelopes of this phase, which tend to form around the SiC fibers, have been found to crack during consolidation.
From observations and analysis that has been made, it appeared that modification of the phase distributions which the alloy forms in the matrix could contribute toward inhibiting matrix cracking and could result in property improvement. From our study it appeared that such property improvement might be achievable by modification of the plasma processing used in forming the matrix of the composite structure.